The human spinal column 10, as shown in FIG. 1, is comprised of a series of thirty-three stacked vertebrae 12 divided into five regions. The cervical region includes seven vertebrae, known as C1-C7. The thoracic region includes twelve vertebrae, known as T1-T12. The lumbar region contains five vertebrae, known as L1-L5. The sacral region is comprised of five vertebrae, known as S1-S5, while the coccygeal region contains four vertebrae, known as Co1-Co4.
FIG. 2 depicts a superior plan view of a normal human lumbar vertebra 12. Although human lumbar vertebrae vary somewhat according to location, they share many common features. Each vertebra 12 includes a vertebral body 14. Two short bones, the pedicles 16, extend backward from each side of the vertebral body 14 to form a vertebral arch 18.
At the posterior end of each pedicle 16, the vertebral arch 18 flares out into broad plates of bone known as the laminae 20. The laminae 20 fuse with each other to form a spinous process 22. The spinous process 22 serves for muscle and ligamentous attachment. A smooth transition from the pedicles 16 to the laminae 20 is interrupted by the formation of a series of processes.
Two transverse processes 24 thrust out laterally on each side from the junction of the pedicle 16 with the lamina 20. The transverse processes 24 serve as levers for the attachment of muscles to the vertebrae 12. Four articular processes, two superior 26 and two inferior 28, also rise from the junctions of the pedicles 16 and the laminae 20. The superior articular processes 26 are sharp oval plates of bone rising upward on each side of the vertebrae, while the inferior processes 28 are oval plates of bone that jut downward on each side.
The superior and inferior articular processes 26 and 28 each have a natural bony structure known as a facet. The superior articular facet 30 faces upward, while the inferior articular facet 31 (see FIG. 3) faces downward. When adjacent vertebrae 12 are aligned, the facets 30 and 31, capped with a smooth articular cartilage, interlock to form a facet joint 32, also known as a zygapophyseal joint.
The facet joint 32 is composed of a superior half and an inferior half. The superior half is formed by the vertebral level below the joint 32, and the inferior half is formed by the vertebral level above the joint 32. For example, in the L4-L5 facet joint, the superior half of the joint 32 is formed by bony structure on the L5 vertebra (i.e., a superior articular surface and supporting bone 26 on the L5 vertebra), and the inferior half of the joint 32 is formed by bony structure on the L4 vertebra (i.e., an inferior articular surface and supporting bone 28 on the L4 vertebra).
An intervertebral disc 34 between each adjacent vertebrae 12 permits gliding movement between the vertebrae 12. The structure and alignment of the vertebrae 12 thus permit a range of movement of the vertebrae 12 relative to each other.
Back pain, particularly in the “small of the back” or lumbosacral (L4-S1) region, is a common ailment. In many cases, the pain severely limits a person's functional ability and quality of life. Such pain can result from a variety of spinal pathologies.
Through disease or injury, the laminae, spinous process, articular processes, or facets of one or more vertebral bodies can become damaged, such that the vertebrae no longer articulate or properly align with each other. This can result in an undesired anatomy, loss of mobility, and pain or discomfort.
For example, the vertebral facet joints can be damaged by either traumatic injury or by various disease processes. These disease processes include osteoarthritis, ankylosing spondylolysis, and degenerative spondylolisthesis. The damage to the facet joints often results in pressure on nerves, also called “pinched” nerves, or nerve compression or impingement. The result is pain, misaligned anatomy, and a corresponding loss of mobility. Pressure on nerves can also occur without facet joint pathology, e.g., a herniated disc.
One type of conventional treatment of facet joint pathology is spinal stabilization, also known as intervertebral stabilization. Intervertebral stabilization prevents relative motion between the vertebrae. By preventing movement, pain can be reduced. Stabilization can be accomplished by various methods. One method of stabilization is spinal fusion. Another method of stabilization is fixation of any number of vertebrae to stabilize and prevent movement of the vertebrae.
Another type of conventional treatment is decompressive laminectomy. This procedure involves excision of the laminae to relieve compression of nerves.
These traditional treatments are subject to a variety of limitations and varying success rates. None of the described treatments, however, puts the spine in proper alignment or returns the spine to a desired anatomy or biomechanical functionality. In addition, stabilization techniques hold the vertebrae in a fixed position thereby limiting a person's mobility.
Prostheses, systems, and methods exist which can maintain more spinal biomechanical functionality than the above discussed methods and systems and overcome many of the problems and disadvantages associated with traditional treatments for spine pathologies. One example of such prosthesis is shown in FIG. 4. FIG. 4 shows an artificial cephalad and caudal facet joint prostheses 36 and 50 for replacing a natural facet joint. Cephalad joint prosthesis 36 replaces the inferior half of a natural facet joint. Cephalad prosthesis 36 has a bearing element 38 with a bearing surface 40. Caudal joint prosthesis 50 replaces the superior half of a natural facet joint. Caudal prosthesis 50 has a bearing element 52 with a bearing surface 54. Conventional fixation elements 56 attach cephalad and caudal facet joint prostheses 36 and 50 to a vertebra in an orientation and position that places bearing surface 40 in approximately the same location as the natural facet joint surface the prosthesis replaces. The prosthesis may also be placed in a location other than the natural facet joint location.
The spinal column permits the following types of movement: flexion, extension, lateral movement, circumduction and rotation. Each movement type represents relative movement between adjacent vertebra or groups of vertebrae. In addition, these relative movements may be simple movements of a single type but it is more likely that a single movement of the spine may result in several movement types or compound movement occurring contemporaneously. In the illustration of FIG. 4, this translates into movement between the upper vertebral body 12 attached to the cephalad prosthesis 36 and the lower vertebral body 12 attached to caudal prosthesis 50. The movement of the vertebral bodies 12 can result in large, complex forces being generated and transmitted through the prosthesis. The point or points of contact between the bearing surface 40 of the cephalad prosthesis 36 and the bearing surface 54 of the caudal prosthesis 50 can transmit enormous amounts of force onto both the cephalad and caudal facet joint prostheses 36 and 50. The distance between each conventional fixation element 56 and the point or points of contact serves as a lever arm, thereby applying an enormous amount of axial, lateral and torque forces about each of the conventional fixation elements 56, which act as fulcrums. Thus, cephalad prosthesis 36 experiences a force somewhere on bearing surface 40, which is expressed as axial, lateral and torque forces about the conventional fixation element 56 of the cephalad prosthesis 36; and likewise, caudal prosthesis 50 experiences a force somewhere on bearing surface 54, which is expressed as axial, lateral and torque forces about the conventional fixation element 56 of the caudal prosthesis 50. As a result, enormous amounts of such forces can be generated and must be absorbed by the facet joint prostheses and its anchoring system(s).
The existence of enormous amounts of torque presents significant problems for permanent fixation of facet joint prostheses into vertebra. Over time, this torque can act to loosen conventional fixation elements, ruin the facet joint, and require more surgical intervention to restore the facet joint prostheses in the vertebra.
Thus, what is needed is a solution to the torque problem experienced by facet joints of artificial vertebral prostheses.